The present disclosure relates generally to the field of electrical drives. More particularly, the disclosure relates to techniques for pre-charging an AC circuit in a power drive.
In the field of power electronic devices, a wide range of circuitry is known and currently available for converting, producing and applying power to loads. Depending upon the application, motor drive systems may include circuitry that converts incoming power from one form to another as needed by the load. In a typical drive system, for example, rectifier circuitry converts alternating current (AC) power (such as from a utility grid or generator) to direct current (DC) power. Inverter circuitry can then convert the DC signal into an AC signal of a particular frequency desired for driving a motor at a particular speed. The rectifier circuitry and the inverter circuitry typically include several high power switches, such as insulated-gate bipolar transistors (IGBTs), controlled by drive circuitry. Motor drive systems also often include power-conditioning circuitry, including capacitors and/or inductors, which remove undesirable ripple currents on the DC bus.
Sometimes during operation of a motor drive system, and particularly during start-up, the drive circuitry may draw high levels of current due to the charging of power conditioning capacitors on the DC bus. Furthermore, power-conditioning capacitors (e.g., LCL capacitors) on the AC bus may also be susceptible to high current levels. To avoid a high in-rush current during start-up, a typical motor drive system may include pre-charge circuitry that applies a smaller initial current to the AC bus of the drive prior to start-up to charge the DC power-conditioning capacitors before applying a full source voltage. Such techniques may be referred to as AC pre-charging.
Typical AC pre-charging techniques involve circuitry that may not be suitable for pre-charging the power-conditioning capacitors. Furthermore, conventional techniques may not be efficiently sized for AC pre-charging and isolating the motor drive from the AC supply during fault conditions.